Dissociation in molecule-surface collisions

Abstract

Dissociation at surfaces is a very important process, both scientifically and technologically. Dissociative chemisorption of molecules impinging at surfaces with thermal energies forms the basis of many processes, like catalysis. In thermal collisions it is evident that the kinetic energy of the impinging molecules is insufficient to break the molecular bond. Electronic processes occurring in the close proximity of the surface must drive the dissociation. At high translational energies (E>10 eV) both collision-induced dissociation, due to direct momentum transfer, and electronic dissociation are energetically allowed for fast molecules. In the case of negative ion formation, e.g. the formation of O₂^- in O₂ surface scattering, most of the dissociation observed can be attributed to mechanical dissociation. The dissociation has been shown to scale mainly with the normal energy and shows a strong orientation dependence. In case of the formation of positive hydrogen ions in molecule-surface collisions Van Slooten et al. (1992) have found that the degree of dissociation scales with the total energy times the total scattering angle squared. This points to the importance of dissociation induced by a collision with a single substrate atom. At high translational energies both collision-induced dissociation, due to direct momentum transfer, the electronic dissociation are energetically allowed.